1. Field of the Invention
The present invention relates generally to pressure transducers and, more particularly, to pressure transducers which incorporate stress reducing configurations to permit the measurement of extremely low pressures while avoiding measurement errors which can be caused by stresses that result from temperature changes or external forces exerted on the transducer.
2. Description of the Prior Art
Many different types of pressure transducers are known to those skilled in the art. One particular type of pressure transducer utilizes a silicon substrate having a diaphragm portion that is flexible and responsive to pressure on the surface of the diaphragm. In these types of pressure transducers, it is very well known to provide a plurality of piezoresistive devices on the diaphragm. These piezoresistive devices are usually connected in a bridge arrangement so that changes in resistance of the devices can be electrically determined from stress induced changes in resistivity of the piezoresistive devices. When the diaphragm of the substrate is deformed in response to the pressure on the diaphragm, the piezoresistive devices experience a stress which alters their resistive characteristics. Measurement of the resistance of the piezoresistive devices permits the magnitude of deformation of the diaphragm to be determined which, in turn, allows the pressure on the diaphragm to be calculated. If the pressure transducer is intended for use in measuring differential pressure, a first pressure is exposed to one side of the diaphragm and a second pressure is exposed to the other side.
U.S. Pat. No. 4,333,349, which issued to Mallon et al on Jun. 8, 1982, describes a particular balancing network for use in association with a piezoresistive semiconductor bridge configuration of the type which is commonly used in pressure sensors. The balancing network comprises a plurality of series resistors connected to the sensing elements in the bridge configuration. Each of the resistors differs from the previous one according to a power of two in order to form a binary ladder arrangement. The individual resistors are associated with terminals to allow the transducer manufacturer to selectively short one or more resistors to provide zero balance compensation. The resistors are located on the nonactive portion of the semiconductor substrate and are fabricated by the same techniques employed for fabrication of the semiconductor piezoresistive sensing elements to assure temperature tracking of the unit with the desired temperature operating range. Although this particular patent describes a specific binary balancing apparatus, the basic concept of employing a plurality of sensing elements, or piezoresistive devices, is very well known to those skilled in the art.
U.S. Pat. No. 4,314,225, which issued to Tominaga et al on Feb. 2, 1982, discloses a pressure sensor which has a semiconductor diaphragm, such as a silicon diaphragm, that is formed with at least one diffused resistor in a surface region on one side thereof. A silicon block, which has the diaphragm, is bonded to the inside of a box-like package such that the diffused resistor is exposed in a vacuum chamber defined in the package and that a fluid pressure can arrive at the back side of the diaphragm through a hole of the package. To minimize unwanted straining of the silicon diaphragm by thermal influences, the package is made of a material such as Mullite whose linear expansion coefficient is close to that of silicon. To prevent an accidental change in the output characteristics of the sensor by the influence of an unintended external force, the package is supported above a base plate by pillar-like lead frames and confined in a space provided by fixing a cap to the base plate. A pressure introduction pipe is attached to either the base plate or the cap.
U.S. Pat. No. 4,295,117, which issued to Lake et al on Oct. 13, 1981, describes a pressure sensor which comprises a silicon chip which has a diaphragm formed therein with piezoresistive strain responsive resistors. The pressure sensor element is mounted in a housing of molded polyester material having a much different temperature coefficient of expansion than silicon chip. The silicon chip is protected from thermally induced and other stresses by a mounting arrangement comprising a glass base secured to the housing by a soft adhesive which largely prevents stress being transmitted from the housing to the base. The base has a short pedestal on which a glass die is mounted which, in turn, supports the silicon chip. The short pedestal adds localized rigidity to the base so that any stresses in the base are taken up outside the region of the pedestal. One face of the glass die is bonded to the base by a relatively soft epoxy to further inhibit stress transmission and the silicon chip is securely bonded to the opposite face of the die. Contacts on the silicon chip are wire bonded to conductors which are insert molded into the housing.
U.S. Pat. No. 4,287,501, which issued to Tominaga et al on Sep. 1, 1981, describes a pressure sensor that comprises a pair of semiconductor diaphragm blocks in which each block has a diaphragm on a front surface of which a diffused resistor is formed as a pressure sensitive element. They are confined within a sealed hollow package and bonded at their circumferential supports to opposite inside surfaces of the package such that the front surfaces of the diaphragms are positioned within a vacuum space within the package. This package is provided with holes through which fluid pressures subject to measurement are introduced so as to arrive at the back surfaces of the diaphragms .
U.S. Pat. No. 4,276,533, which issued to Tominaga et al on Jun 30, 1981, describes a pressure sensor assembly which includes a silicon diaphragm block that has a diaphragm at an eccentric position thereof with a diffused resistor formed as a pressure sensitive element on front surface of the diaphragm. A silicon support plate is bonded to the diaphragm block so as to cover a back surface of the diaphragm. A silicon support plate is bonded to the diaphragm block so as to cover a back surface of the diaphragm. The diaphragm assembly is accommodated within and bonded to the inside of a hollow package at an end portion of the assembly remote from the diaphragm in the direction of extension of the bonded surface of the block and the support plate. The front surface of the diaphragm and the diffused resistor thereon are exposed to a vacuum while a fluid pressure subject to measurement is introduced through a passage extending through the package and the end portion of the assembly bonded to the package so as to arrive at a back surface of the diaphragm.
Many of the pressure sensors known to those skilled in the art employ techniques to reduce the sensitivity of the pressure sensor to stresses which are either induced through temperature changes or external forces exerted on the sensor. However, the problem of protecting a pressure sensor from stress is significantly exacerbated when the pressure sensor is used in association with an amplification and compensation circuit within the same housing structure. In most situations, the amplification and compensation circuit is disposed on a substrate, such as a ceramic plate, which is connected in electrical communication with both the piezoresistive devices on the diaphragm of the pressure sensor and terminals which permit electrical signals and electrical power to pass between the circuit on the ceramic plate and a device which is external to the pressure transducer. In most cases, the housing package has a coefficient of thermal expansion which is significantly different than that of the ceramic plate which, in turn, has a coefficient of thermal expansion which is significantly different than that of the diaphragm. Therefore, changes in temperature are likely to cause unequal changes in size for the various components of the pressure transducer which, in turn, causes stresses in the diaphragm which can result in inaccuracies and inconsistencies. In addition, the pressure transducer package is usually mounted to another device, such as a printed circuit board, for use in a larger system. When the package is mechanically attached to the other device, the attachment process can induce additional stresses in the package which can be transmitted to the silicon diaphragm and its piezoresistive devices. All of these factors can lead to improper operation of the pressure transducer.
When the pressure transducer is intended for use in measuring very low pressures, the problems described above are significantly more disadvantageous. Therefore, it would be beneficial if a pressure transducer with an amplification and compensation circuit is provided in a way which reduces the effect of temperature induced stresses and stresses caused by external forces.